This invention concerns a device for controlling an automatic transmission, in particular of a CVT, which is connected with a pendulous drive, especially an internal combustion engine, for a motor vehicle having front-transverse drive.
A CVT is a transmission having a variator for continuous adjustment of the ratio. A known design is a belt-drive variator with a first pulley cone pair upon an input shaft and with a second pulley cone pair upon an output shaft, each pulley cone pair consisting of a first cone stationary in the axial direction and a second cone movable in the axial direction (primary and secondary cones, respectively). Between the pulley cone pairs rotates a belt-drive element, such as a pushing linked band. The primary and secondary cones are adjusted by a pressure medium with the aid of an electronic control unit which, via electromagnetic actuating elements and hydraulic valves, controls the pressure level of the actuating areas of primary and secondary cones.
DE-OS 42 34 103 has disclosed a device for controlling the pressure of the primary cone. The device comprises an electromagnetically controlled primary valve and a hydraulic secondary valve. The primary valve is controlled by an electronic control unit. If the line from electronic control unit to the primary valve is interrupted, the device changes over to emergency operating mode. The pressure level of the primary pulley is determined here by the secondary valve. The position of the secondary valve, in turn, is determined via a mechanical clutch of the primary pulley. This known device has the disadvantages of not covering a complete failure of the electronic control unit.
The Applicant""s older German patent application No. P 44 36 506.3 proposed a device for control of a CVT driven by a drive unit in which an electronic control unit determines, via electromagnetic actuating elements and hydraulic valves, the pressure level of the actuating areas, particularly not dynamically balanced, of primary and secondary pulleys and where in case of failure of the electronic control unit an emergency device is activated which has two pressure-regulating valves, two pressure-reducing valves and at least one emergency valve wherein a primary valve and a secondary valve adjust a constant, static pressure ratio and force ratio between primary pulley and secondary pulley during constant static secondary pressure level. By the amount of the static force ratio and of the dynamic forces on the pulley, it is, possible to establish the extent of the ratio change of the CVT from normal operation to emergency operating mode. By emergency operating mode is to be understood the state of a CVT in which no rotational speed regulation and no contact pressure regulation of the secondary pulley occur.
In this known device, the ratio of the CVT changes according to the generated torque of a drive unit, especially of an internal combustion engine, the variable ratio change in emergency operating mode making a reliable start on mountain and an improved driveability at maximum speed possible. As input signal are used, for example, the signal of a load position of the drive unit (throttle valve position in case of an internal combustion engine), the rotational speed of the transmission input shaft, the rotational speed of the output shaft, or the temperature of a pressure medium. An electronic control unit determines from the input signal an operating point and adjusts the appertaining rotational speed value of the transmission input shaft or the ratio of the CVT. If an error occurs, the CVT changes over to emergency operating mode in which there is no longer any rotational speed regulation and any contact pressure regulation of the secondary pulley.
In the emergency operating mode, a pump conveys pressure medium to a first and a second pressure-reducing valve which are respectively connected with a first and a second electromagnetic pressure-regulating valve. Both electromagnetic pressure-regulating valves are, in turn, connected with a first and a second emergency valve, the latter being connected with a primary valve and a secondary valve which, via corresponding lines, are directly connected with the pump. The primary valve determines the pressure level of the actuating area of the primary pulley and the secondary valve determines the pressure level of the actuating area of the secondary pulley. The shifting positions of both emergency valves are determined by the first electromagnetic pressure-regulating valve.
In this known device, however, the need for locating the contact pressure for the corresponding pulleys, according to extreme of the driveability range, is also disadvantageous. Thus, for the capability of the variator of transmitting at low ratio and great input torque, a higher contact pressure on the secondary pulley is required. Subject to the draft of the constant, static pressure ratio between primary pulley and secondary pulley and the selected control of the emergency valves, the adjusted static force on the secondary pulley is constant in the whole torque range, the magnitude depending on the maximum input torque to be transmitted. But in all other operating modes, this results in an overpressure which acts disadvantageously upon the operating temperature with increased loss of efficiency and upon the wear and tear.
EP-A 584 457 has made known a device for regulating the coupling and uncoupling of the master clutch in the transmission of a driving device of a motor vehicle consisting of an engine and an electrohydraulically actuatable automatic transmission wherein to determine the bracing between the driving device and the body, the displacement extent of the input device is measured against the body and fed to a transmission control device which, in specific driving situations, adjusts the control pressure in the master clutch of the transmission in a manner such that the master clutch begins to grind. Instead of the displacement, the extent of torque of the transmission or the information of the supporting force in the step bearings of the driving device may be used. The transmission is constructed as electrohydraulically controllable automatic transmission actuatable, via an electrohydraulic control device, the same as from a sensor for measuring the force in the step bearings for the driving device. With this known device, a master clutch can be exclusively coupled and uncoupled, for example, to reduce shaking when the vehicle is stationary, when gear is introduced and when input wheels are braked.
The invention is based on the problem of providing an emergency drive device for the electrohydraulic, or also only hydraulic control of a CVT transmission, using an input signal equivalent to an engine torque with which the above mentioned overpressing of the variator pulleys is prevented.
It is proposed, according to the invention, that torque signals of the input unit serve for adjusting a variable pressure level during constant, static pressure ratio between primary and secondary pulleys adjusted by the hydraulic transmission control. The invention is based on the knowledge that the torque generated by the drive unit, that is, the internal combustion engine, must be reinforced. On the reinforcement, by means of a torque support, a signal equivalent to the real input torque can be measured off, that is, a signal equivalent to an engine torque. A preferred example of utilization are aggregate combinations of internal combustion engine and CVT transmission in front-transverse arrangement with pendulous support. The supporting torque can be directly measured on the torque support, e.g. by means of a pressure sensor or a displacement path sensor.
The invention is also based on the knowledge that, in the emergency operating mode, the signal equivalent to the engine torque must be available without limitation, i.e. even in case of complete failure of the electronic control. Especially suitable for the transmission of the engine-torque equivalent signal to the transmission control are hydromotors which can be designed as linear engines or rotary engines and are directly placed on the torque support (for example, the transmission support) of the internal combustion engine and are hydraulically connected with the transmission control.
With the torque-equivalent support torque is modulated the pressure level on the primary and secondary pulleys of the CVT while maintaining the hydraulically established, constant, static pressure ratio.
Subject to the engine-torque equivalent modulation of the pressure level on the secondary pulley in the emergency operating mode, the overpressing can be considerably reduced specifically to a safety value which takes into account the dynamic drive line effects and the tolerances specific to the transmission.
The lowering of the pressure level in the emergency operating mode has a positive influence especially on the temperature level of the CVT, since formerly the enormous overpressure appearing at extreme values of the driveability range in the cooling location. Another positive influence of pressure lowering is the reduced wear of the variator.
One other advantage of the inventive device is the possible use of the engine-torque equivalent pressure also outside the emergency operating mode. The engine-torque equivalent signal can thus be used for direct loading of the secondary pulley and of the clutch valve in order to directly convert hydraulically drive line dynamics. Special advantages here are in the availability of an engine-tolerance dependent size and thereby possible lowering of the overpressure, i.e. the reduction of the safety additions, with the consequence of improvements in efficiency of the transmission.